Precipitated calcium carbonate is used in a wide variety of end uses. In some end uses, the brightness of the precipitated calcium carbonate is of minor importance. However, in a number of end uses e.g. use of precipitated calcium carbonate as a filler, as a coating agent or as a pigment, it is important or critical that the precipitated calcium carbonate have a high degree of brightness. As used herein, brightness of calcium carbonate is measured by the method of TAPPI T646 om-94. Use of precipitated calcium carbonate as a filler, coating agent or pigment frequently requires that the brightness be not less than 95%.
A number of techniques may be used to obtain precipitated calcium carbonate. A typical method involves forming a suspension of lime in water, converting the lime into calcium hydroxide, and treating the resultant suspension of calcium hydroxide with carbon dioxide so as to form calcium carbonate. Such treatment leaves grit, colored material and other insoluble matter in the suspension, which contaminates the resulting PCC that is formed. Thus, in order to obtain PCC with acceptable properties, the lime used in the process must be of a high purity.
Other techniques are disclosed in the following patents. Canadian Patent 441 583 of W. P. Foster discloses carbonation of lime suspensions in the presence of 0.125% by weight of ammonia or amines, optionally using ammonia or amines in the form of one of its compounds, such as chloride or stearate, as such compounds react with a small part of the lime to liberate ammonia or amine.
Canadian Patent 352 149 of W. H. MacIntire discloses a process for forming a double precipitate of calcium and magnesium carbonate by adding ammonia gas to a suspension of dolomite and then mixing with carbon dioxide.
Canadian Patent 2 066 655 relates to a process of preparing calcium carbonate in which a reagent that is a polybasic acid, a protein or an amine is added to an aqueous suspension of slaked quick lime, and calcium carbonate is precipitated with carbon dioxide. The amount of the reagent is preferably 0.01 to 15% based on the weight of dry calcium oxide.
Canadian Patent 2 090 088 discloses a method of producing relatively pure calcium carbonate from an impure source of calcium oxide in which a slurry of aqueous hydrated lime is treated with a water-soluble source of anions that are capable of forming a salt with calcium ions that has a dissociation constant greater than the dissociation constant of calcium oxide, for example ammonium nitrate or ammonium chloride.
U.S. Pat. No. 3,347,624 discloses a process for the manufacture of calcium carbonate in which 0.001 to 0.1 grams, per gram of calcium oxide, of a water soluble of diamine is added to an aqueous suspension of calcium oxide and then carbonated with carbon dioxide.
U.S. Pat. No. 5,871,519 relates to a process for producing magnesium oxide and/or hydrates thereof from predominately magnesium or magnesium-calcium raw materials in which calcium oxide is dissolved from the mixture using an aqueous solution containing an organic amine and a salt formed from an organic amine and an acid capable of forming a soluble calcium salt with the organic amine.
U.S. Pat. No. 5,232,678 relates to the preparation of calcium carbonate in which 0.01 to 15% by weight of an amine is added to the suspension of the quick lime.
If an ammonium salt e.g. ammonium nitrate, is used to dissolve or complex the lime into solution, and if carbon dioxide and air is used in order to precipitate the calcium carbonate, then a significant part of the cost of the process is the capital and operating cost of the scrubbing of ammonia from the exit gas, which is a mixture of ammonia, carbon dioxide and air.